1. Field of the Art
This invention is in the field of test apparatus, particularly the apparatus used in the field of aerodynamics and more particularly in determining and indicating characteristics of airflow over and along surfaces, such as the surfaces of aircraft, automobiles, boats, buildings and the like. More specifically the subject apparatus relates to the technique known in the art as tufting. So called tufts are attached to the areas of interest of the surface and the orientations and motions of the tufts caused by the airflow can be observed and photographed to provide information about the directions and states of the flow over the surface.
2. Prior Art
While tufting has been practiced for many years, the apparatus has remained rather rudimentary for most of that time. Briefly, lengths of flexible material such as string, thread, twine, yarn, cord, filament and the like are attached at one end to the test surface and the material tends to align with passing air. Various lengths and kinds of material are used, along with various methods of fastening the tufts to the surface, the selection of lengths, materials and fastening techniques being based on experience and experiment. The technique is used on full scale testing and in testing of models, especially wind tunnel models.
To be particularly effective on models the tufts must be small and they are then difficult to see and photograph. This problem is addressed and solved by technology developed by the subject inventor. The technology involves the use of small diameter fluorescent filaments as tufts. The tufts then are made more visible by illuminating them with light of the appropriate wave lengths to cause the fluorescense. This technique enhances the visibility and photographability of the tufts sufficiently to allow the use of very fine, flexible filaments and hence significantly enhances the use of filamentary tufts on models. This technology was published in the November, 1980 issue of Astronautics and Aeronautics.
The effectiveness of the use of tufts on larger scale equipment such as full scale airplanes is also limited to some extent by difficulties with the visibility of the tufts. Furthermore, the effectiveness is limited by the fluttering or waving of the tufts under certain conditions. The waving and fluttering involves the same physical phenomena as the waving of flags and the like. In attempts to improve visibility, larger diameter and longer tufts have been used. However, the larger and/or longer tufts seem to aggravate the tendency for self excited fluttering and present other problems. They tend to interfere too much with the flow being assessed. The fluttering makes them more difficult to attach satisfactorily, adding to the already time-consuming and relatively expensive process of installing the tufts. Also, the fluttering and waving limits the durability of the tufts.
The subject invention addresses the problems associated with visibility, fluttering, durability and installation difficulties and expense.
The closest known prior art to the invention comprises conically shaped drogues as shown in U.S. Pat. Nos. 3,251,565 and 3,310,257. Drogues are used on lines, hoses, antennas and the like trailing from aircraft to prevent whipping and flapping of the trailing elements. As is well known in the art, drogues are effective because of the drag they apply to the trailing element and, to produce sufficient drag, the cone angles are relatively large, in the range of 30.degree. to 90.degree.. For the purpose of the subject invention the drag of drogues at the end of the tufts would add to the problems of attachment of tufts and the drogues would have to be minute. Also, there is experimental evidence to the effect that such an application of drogues would not solve the described problems.
It is evident to those skilled in the art that tufts function, in effect, as three dimensional wind vanes, in contrast to the two dimensional wind vanes known in general as weather vanes, wind vanes, flow direction indicators and the like. The wind vanes which are the closest known prior art to the subject invention are the vanes of the type used on angle of airflow sensors such as the apparatus marketed by Rosemount, Inc. Minneapolis, Minnesota. The vane of such sensors is at the end of a pivoted arm and is wedge shaped with the sharp edge of the wedge facing into the wind (airflow). The angle between the faces of the wedge is on the order of 10.degree. and this configuration has proven to be very stable over the full range of air flow conditions such vanes are subject to when mounted on the surfaces of airplane fuselages.